Many optical instruments and optical systems include imaging systems that gather light from an object and direct the light into the human eye to enable the eye to perceive an image of the object. Desirable properties of such imaging systems include high image brightness, high image resolution, low geometric and chromatic aberrations, low distortion, a wide field of view, and an adequate and non-critical eye relief. It is also desirable that an imaging system be simple and easy to manufacture, and that any adjustments of its components be simple and easy to perform. Finally, it is desirable that the imaging system be capable of adjustment to provide sharply-focused images for users with vision defects such as myopia and presbyopia. Meeting the above-stated performance criteria is relatively easy when no constraints are imposed on size and weight. Many different optical arrangements capable of meeting the above-stated performance criteria are known. Recently, a market demand has arisen for wearable displays, especially for wearable displays having a configuration similar to a pair of eyeglasses. Such displays are called eyeglass displays. An eyeglass display may be connected to a portable computer, a portable television receiver, a portable Digital Versatile Disk (DVD) player, or other generator of a video signal to provide the user with a highly private, high-quality display of data or entertainment. An eyeglass display may also form part of a virtual reality system.
To maximize user comfort and for aesthetic reasons, an eyeglass display should have the lowest-possible weight and bulk. To reduce the weight and bulk of the eyeglass display, the display element that generates the pictures for viewing by the user is very small and is located much closer to the user's eye than a comfortable viewing distance. Consequently, the user cannot view the display element directly. Instead, an optical imaging system must be interposed between the display element and the user's eye. The optical imaging system gathers light from the display element and directs the light into the user's eye. The optical imaging system forms a magnified image of the display element at a comfortable viewing distance from the user's eye. The magnification provides the image an apparent size comparable with that of a large-screen computer monitor at a conventional viewing distance (about 50 cm). To minimize the weight and bulk of the eyeglass display, the weight and bulk of the optical imaging system must also be minimized. Requiring that the weight and bulk of the optical imaging system be minimized and that the optical imaging system also be very simple to manufacture greatly increases the challenge of designing an optical imaging system that additionally meets the performance criteria set forth above.
Other types of miniature display, such as electronic viewfinders for electronic still and motion picture cameras, have performance requirements similar to eyeglass displays.
FIG. 1 shows a plan view of an example of a conventional optical imaging system 10. The optical imaging system 10 is used in a helmet-mounted display system and performs substantially the same function as the optical imaging system of an eyeglass display system. The optical imaging system 10 is composed of the plane mirrors 12 and 14, the 50--50 beam splitter 16, the concave mirror 18 and the four lenses 20, 22, 24 and 25. The image plane 26 is laterally displaced from the user's eye located at or near the exit pupil 28. Light diverging from the image plane passes through the lens 20, reflects from the mirror 12, passes though the lenses 22, 24 and 25, is reflected by the mirror 18, is partially reflected by the beam splitter 16, is reflected and converged by the concave mirror 18 and is transmitted through the beam splitter to the exit pupil. This arrangement forms an upright, enlarged image of the object located at the image plane. To meet the performance requirements set forth above, the eight major components constituting the optical imaging system 10 must be accurately located relative to one another in terms of both position and rotation about at least two axes. Moreover, the lenses 20 and 22 are preferably doublets, as shown, to reduce chromatic aberration.
The conventional optical imaging system 10 has a number of performance shortcomings. It has limited field of view. When the object located at the image plane 26 is a display element that has to be externally illuminated, the bulk of the optical imaging system has to be substantially increased. If the display element is a transmissive display element, the back illumination system that illuminates the display element increases the bulk of the optical imaging system. If the display element is a reflective display element, the need to provide sufficient clearance between the display element and the lens 20 to accommodate the illumination system increases the bulk of the optical imaging system. The conventional optical imaging system is incapable of accommodating compact reflective illumination optics, and separate illumination optics must be provided. Finally, the conventional optical imaging system locates the display element at a point level with the user's eye, but horizontally displaced therefrom by a substantial distance. The resulting width of the optical imaging system causes the eyeglass display to obstruct a substantial portion of the user's peripheral vision.
Other known optical imaging system suffer from similar or additional problems.
What is needed, then, is an optical imaging system that has the lowest-possible weight and bulk, and that forms a high-resolution, low-distortion image of an object and locates the image at a comfortable viewing distance from the user=s eye. When used in an eyeglass display, the optical imaging system should form an image having an apparent size comparable with that of a large-screen computer monitor at a conventional viewing distance, and should locate the image at a comfortable viewing distance. The optical imaging system should also be capable of operating with both reflective and transmissive display elements.